The manufacture and/or testing of high voltage (HV) capacitors used in high voltage power transmission utilizes processes that in many respects can be improved. HV capacitors are typically very heavy and bulky; an exemplary HV capacitor weighs 50 Kg and is 2 meters long. In one variant, HV capacitors can be configured for use as a CVD (Capacitor Voltage Divider).
The manufacture of HV capacitors typically includes the assembly of a series string or stack of capacitor cells, which are subsequently inserted into an open capacitor housing. In the prior art, individual capacitor cells are joined in series by means of the introduction of additional material, which is used to form of a bond between the cells. FIG. 6b illustrates bonding of two aluminum foils 10 of respective capacitor cells by use of additional material 12, for example a solder, a conductive glue, a joining tab, etc. As represented by dashed lines in FIG. 6b, the additional material may act to deform an electrical field that is formed when an electric potential is present across the aluminum foils; degraded performance may be one consequent result. In the prior art, after insertion of cells in an unsealed housing, the housing is placed into a large oven chamber. With oven closed, the capacitor housing and capacitor cells within are subjected to one or more cycle of vacuum and/or high temperature so as to remove moisture from the cells and the interior of the housing. Increased oven drying throughput may be achieved by drying more than one HV capacitor housing (a batch) at a time, but the oven size needs to be increased accordingly. For example, in order to accommodate a batch of 125 HV capacitor housings, in one embodiment an interior of a drying oven is dimensioned to be on the order of about 3×5×5 meters. Although large ovens can permit a large number of capacitor housings to be dried at one time, a large amount of unused free volume remains within the oven, which requires that more air be evacuated and/or more air be heated to maintain a given temperature or vacuum within the oven; increased drying time and/or increased energy usage may be a consequent result.
After drying, the HV capacitor housings are physically removed from the oven for impregnation. The unsealed HV capacitor housings are removed from the oven and immersed or filled in their entirety in a vat or tank of impregnation fluid so as to fully impregnate the interior of the housings and capacitor cells therein. After the impregnation step, each capacitor housing is individually fitted and sealed with sealing end caps. Each sealing end cap may include terminals, with which external electrical access to the capacitor cells within the housing may be made.
In the prior art, impregnation of HV capacitors, whether individually or as a batch, is a very dirty and messy process that leaves residues of impregnation fluid on the exterior of each capacitor housing, as well, about the surrounding environment. Consequently, after sealing of a capacitor housing with sealing end caps, impregnation fluid typically needs cleaned from the housing exterior and other exposed apparatus. After impregnation and cleaning, the HV capacitor housings are reinserted into the oven, the temperature of which is raised again so as to increase the temperature of the impregnation fluid within the sealed housings. The increased temperature increases pressure within the now sealed capacitor housings. After an extended period of time, the HV capacitor housings are removed from the oven and inspected for leakage of impregnation fluid, particularly at sealed electrical connection points and end caps. If no leaks are detected, the HV capacitors are tested under application of a high voltage, and if the HV test is passed, the HV capacitors can be made available for use.
Variations in the order of testing, heating, and impregnation to that described above may exist in the prior art, but have in common that during each movement, test, and dis/assembly step, the HV capacitors and cells are exposed to impurities, moisture, and other undesired materials. The undesired materials may to some extent be reduced by extra time consuming drying and vacuum steps but, nevertheless, are always present. Performance of prior art capacitors is consequently negatively affected.
It is desired to improve upon one or more aspects of the prior art.